Avoid reading from Viper for path and URL funcs

The gain, given the "real sites benchmark" below, is obvious:

```
benchmark           old ns/op       new ns/op       delta
BenchmarkHugo-4     14497594101     13084156335     -9.75%

benchmark           old allocs     new allocs     delta
BenchmarkHugo-4     57404335       48282002       -15.89%

benchmark           old bytes       new bytes      delta
BenchmarkHugo-4     9933505624      9721984424     -2.13%
```

Fixes #2495

hugolib/hugo_sites_test.go

@@ -35,6 +35,7 @@ func testCommonResetState() {
 	hugofs.InitMemFs()
 	viper.Reset()
 	viper.SetFs(hugofs.Source())
+	helpers.ResetConfigProvider()
 	loadDefaultSettings()
 
 	// Default is false, but true is easier to use as default in tests